1. Field
The subject invention is in the field of mechanical power transmission and transfer mechanisms, particularly the field of power transfer from an engine in a boat to the propeller driven by the engine. More particularly it is in the field of such drives in which the drive shaft and propeller shaft are parallel and essentially horizontal such as in well known inboard/outboard drives in which power is transmitted through the stem of a boat above the waterline and then down to the propeller shaft and propeller. However, the subject drive is an inboard drive which is mounted through the bottom of the boat to transmit the power. But a transom mounting is also possible. A feature common in inboard/outboard drives is that the drive shaft and propeller are parallel and power is transmitted between the two using bevel and/or miter gearing, chains or belts. One important objective of such drives is that the components in the water present as low frontal area as possible to minimize drag losses. This is particularly important for sailboats in which the propeller is an auxiliary power source and must present minimum drag when the boat is under sail. The problem is more severe for larger boats in which auxiliary power levels are in the range of 100 to 300+H.P. Since such boats are not high speed boats, propeller speeds must be relatively low and propellers fairly large to achieve satisfactory efficiency. These facts require that the torque capacity of the drive be high relative to the horsepower level. In the stated horsepower range high torque per horsepower gear drives become bulky and very complex. Chain drives are better suited to high torque per horsepower transmissions; however good operation, efficiency and long life of chain drives, particularly bi-directional drives, requires that the chains be under tension and correspondingly free of slack and running in a straight line from sprocket to sprocket. It is close to physically impossible and economically and practically impossible, using conventional techniques, to design and assemble a chain drive in which the chain is always in tension without using some sort of tensioning device. This is caused in large part by the fact that use invariably involves wearing in and wear will allow the chain(s) to go slack. Tensioning devices inherently tend to add undesirable amounts of frontal area and complication.
2. Prior Art
There is much prior art in the particular field described above and many of the prior art drives use chains. The U.S. patents listed here are typical examples:
1. U.S. Pat. No. 2,809,605 7. U.S. Pat. No. 4,887,983
2. U.S. Pat. No. 3,403,655 8. U.S. Pat. No. 4,925,413
3. U.S. Pat. No. 3,795,219 9. U.S. Pat. No. 4,932,907
4. U.S. Pat. No. 3,951,096 10. U.S. Pat. No. 4,992,066
5. U.S. Pat. No. 4,645,463 11. U.S. Pat. No. 5,813,887
6. U.S. Pat. No. 4,869,692 12. U.S. Pat. No. 5,961,358
As background to discussion of this prior art, it is important to state that the chains having the highest power capacity for their size and weight are chains known as silent chains. These chains comprise pluralities of flat links having a tooth form at each end. The side-by side links are pinned end to end so that the teeth forms form teeth when the chain is in contact with the sprockets on which it is mounted. Chain width is determined by the power required and the resulting number of links pinned side-by-side. Making and using these chains as mechanically efficiently as possible results in their having the characteristics that (1) they allow only limited bending in the direction away from the toothed side of the chain, and (2) the durability and efficiency depend on their being as straight as possible between sprockets at all times. These factors relate to the need for tension adjustment and prohibit techniques using tension idlers which would not allow the chain to be straight between sprockets.
Regarding the cited prior art, patents 2,5,6,7,8,9 and 10 utilize roller chains and show no specific means for adjusting tension except in patents 1 and 10. The adjustment in patent 1 is accomplished by adjusting the distance between the drive and driven shafts by having the shafts in separate assemblies which fit together telescopically. It is judged that making the housings telescopic is not an economically efficient technique and is mechanically cumbersome. Also to make such a design oil tight is very complex. The means shown in patent 10 comprises an oval shaped cam pivotally mounted midway between the lengths of chain between the sprockets. Rotating the cam in one direction so that the cam ends contact the chain will spread the chain apart and increase tension. This does not allow the chain in tension to be straight. Also, this means can only be used in unidirectional drives. With rotation in the wrong direction the cam would be forcefully rotated into the chain and jammed.
The remaining patents show drives using belts of some kind. Patent 2) uses a toothed belt and shows no means of adjusting tension even for the purpose of removing and installing the belt. Patent 3) also shows the use of a toothed belt with means for adjusting tension for the purpose of removing and installing the belt but none for compensating for belt stretch and other factors which are known to cause loosening of the toothed belts. The means used comprise a spherically mounted bearing on one end of the drive shaft so that when the housing is disassembled and the bearing at the other end of the shaft is removed, the shaft can drop to an angle sufficient to allow the teeth on the belt to clear the rim on the sprocket, thus facilitating removal and replacement. The end of the shaft freed by dismantling the casing is tapered to facilitate its reentry into the bearing when the case is assembled, leveling the drive shaft again and providing nominally acceptable belt tension. However, this adjustment feature does not allow compensation for belt stretch and other factors which are known to cause belt loosening. Also, to enable replacement of this belt the casing is divided vertically into forward and aft parts, generating a need for long parting surfaces and a plurality of fasteners are needed to assemble the casing.
Patent 4) shows the use of dual toothed belts. There are no provisions for tension adjustment and the method of assembly and disassembly of the belt drive is not disclosed.
Many motorcycles transmit driving power from a drive shaft to a driven shaft, the rear axle. Chain tension is adjusted by adjusting the position of the rear axle and everything carried by it relative to the drive shaft. This technique cannot be used in propeller drives because the driven shaft must be enclosed in a housing.
The closest prior art for the subject invention is disclosed in U.S. Pat. No. 6,413,127 xe2x80x9cLow Frontal Area, Inboard Through-Hull Propeller Drive and Methods For Assembling and Adjusting the Drivenxe2x80x9d and U.S. Pat. No. 6,663,449 xe2x80x9cLow Frontal Area, Inboard, Through-Hull Propeller Drivexe2x80x9d and invented by the inventor of the subject invention.
In view of this prior art, the objective of the subject invention is to provide a low frontal area, inboard, through-hull propeller drive for power ranging up to 300+H.P., the drive using a silent chain and having (1) a housing having a minimum number of parts and short parting lines, (2) simple means for adjusting chain tension for installation and removal purposes and compensation for wear without any disassembly, (3) allowing simple chain installation and removal, and (4) using simple conventional sealing techniques.
The subject invention is a low frontal area, inboard, through-hull propeller drive. The drive comprises (1) a casing having a high fineness ratio streamlined cross section shape and an upper and a lower end, (2) a lower end apparatus comprising a propeller shaft, driven chain sprocket and bearings installed at the lower end of the casing through the opening for the propeller shaft and is bearings, (3) an upper end apparatus attached to the upper end of the casing, having a drive shaft, drive sprocket and bearings installed in it, and (4) a silent chain interconnecting the sprockets in the lower and upper apparatus.
The drive shaft and sprocket are carried in the upper apparatus on two bearings, each of which is carried eccentrically in a bearing carrier adjustably installed in a centric bore at an end of the drive casing. The bearing carriers are structurally interconnected and chain tension is adjusted by rotating the interconnected carriers in the centric bores of the upper apparatus. Sealing is provided by O-Rings installed on the bearing carriers which seal against the centric bore of the casing. In this design the forces imposed on the bearing carriersxe2x80x94chain loadingxe2x80x94are directly carried by the housing and not dependent on clamping force of bolts.
The assembly procedure is to install the chain in the lower apparatus and connect the extending chain ends. The upper apparatus is then attached to the upper end and the drive shaft and sprocket are inserted into the casing using the openings at the upper ends of the casing. In a second embodiment the centric bores are in ends of the upper apparatus. Then the bearings and bearing carriers are installed and the bearing carriers are structurally interconnected to coordinate their adjustment movement relative to the casing. The chain tension is adjusted by rotating the interconnected bearing carriers which carry the shaft, sprocket and the bearings which are eccentrically mounted.
The invention is described in more detail below with reference to the attached drawings.